Image forming optical system and electronic instrument using the same

ABSTRACT

An imaging optical system comprises, in order from an object side a first lens having positive refracting power, a second lens having negative refracting power, a concave surface of which is directed toward the object side, a third lens having positive refracting power, a convex surface of which is directed toward an image side and a fourth lens having negative refracting power, wherein the second lens and the third lens are cemented. By such constitution, an angle made by the incident light and the exiting light to the cemented lens can be kept small, and accordingly, the generation of aberrations at the refracting surface and performance change of lenses when these are in a relative decentering position can be suppressed to the utmost.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image forming optical systemwhich can be used by combining with the solid-state imaging element ofCCD, CMOS and the like. And it also relates to an image forming opticalsystem which can be used form a miniature camera and a monitor cameraand the like which are equipped in, for example, a digital still camera,a digital video camera, a cellular phone, PC and the like. Furthermore,the present invention also relates to an electronic instrument such as adigital still camera, a digital video camera, a cellular phone, PC andthe like which use the image forming optical system.

[0003] 2. Description of the Related Art

[0004] In recent years, electronic cameras for taking a photograph byusing a solid-state imaging element like CCD and CMOS instead of using asilver salt film have become popular. In such electronic cameras, for animaging unit which is equipped in a portable type computer or a cellularphone and the like, miniaturization and weight-lightening have beenparticularly demanded.

SUMMARY OF THE INVENTION

[0005] According to the present invention, an image forming opticalsystem with an aperture stop comprises, in order from an object side, afirst lens having positive refracting power, a second lens havingnegative refracting power, a concave surface of which is directed towardan object side, a third lens having positive refracting power, a convexsurface of which is directed toward the image side and a fourth lenshaving negative refracting power, wherein the second lens and the thirdlens are cemented. By such constitution, an image forming optical systemin which performance and size are optimum can be provided.

[0006] According to the present invention, an image forming opticalsystem with an aperture stop comprises, in order from an object side, afirst lens having positive refracting power, a second lens havingnegative refracting power, a concave surface of which is directed towardan object side, a third lens having positive refracting power, a convexsurface of which is directed toward the image side and a fourth lenshaving negative refracting power, wherein the first lens consists ofglass, and the second lens and the third lens are cemented.

[0007] An electric apparatus according to the present inventioncomprises the image forming optical system mentioned above.

[0008] According to the present invention, an image forming opticalsystem in which degradation of performance to manufacture error islittle and high performance is achieved even if it is miniaturized canbe obtained.

[0009] These and other features and advantages of the present inventionwill become apparent from the following detailed description of thepreferred embodiments when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a sectional view showing an optical arrangementdeveloped along the optical axis in the first embodiment of an imageforming optical system according to the present invention.

[0011]FIG. 2 is a sectional view showing an optical arrangement,developed along the optical axis in the second embodiment of an imageforming optical system according to the present invention.

[0012]FIG. 3 is a sectional view showing an optical arrangement,developed along the optical axis in the third embodiment of an imageforming optical system according to the present invention.

[0013]FIG. 4 is a sectional view showing an optical arrangement,developed along the optical axis in the fourth embodiment of an imageforming optical system according to the present invention.

[0014]FIGS. 5A, 5B and 5C are graphs showing spherical aberration,astigmatism and distortion in the first embodiment of an image formingoptical system according to the present invention.

[0015]FIGS. 6A, 6B and 6C are graphs showing a spherical aberration, anastigmatism and a distortion in the second embodiment of an imageforming optical system according to the present invention.

[0016]FIGS. 7A, 7B and 7C are graphs showing a spherical aberration, anastigmatism and a distortion in the third embodiment of an image formingoptical system according to the present invention.

[0017]FIGS. 8A, 8B and 8C are graphs showing a spherical aberration, anastigmatism and a distortion in the fourth embodiment of an imageforming optical system according to the present invention.

[0018]FIGS. 9A and 9B are a front view and a rear view showing anoutlined construction of a cellular phone embodied by an image formingoptical system according to the present invention.

[0019]FIGS. 10A and 10B are a front perspective view and a rearperspective view showing an outlined construction of a digital cameraembodied by an image forming optical system according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Before undertaking the description of the embodiments, reasonswhy the constitution of the present invention has been made as well asfunction and advantages of the present invention will be explained.

[0021] According to the present image forming optical system, an imageforming optical system with an aperture stop comprises, in order from anobject side, a first lens having positive refracting power, a secondlens having negative refracting power, a concave surface of which isdirected toward an object side, a third lens having positive refractingpower, a convex surface of which is directed toward the image side and afourth lens having negative refracting power, wherein the second lensand the third lens are cemented.

[0022] According to the present invention, taking into account of theperformance and miniaturization of the image forming optical system, itis constituted with four lenses that are a first lens having positiverefracting power, a second lens having negative refracting power, athird lens having positive refracting power and a fourth lens havingnegative refracting power.

[0023] It is evident that if the number of the lens which constitutesthe image forming optical system is five or more than five, performanceis improved further. However, the thickness of the lens, the distancebetween lenses and the space of a frame become larger by increasing onepiece of lens. Consequently, large sizing of the image forming opticalsystem cannot be avoided.

[0024] It is difficult that chromatism is compatible with curve of animage surface on optical axis when the number of lens is less thanthree. Even if a performance be secured by using many asphericalsurfaces, it is difficult to manufacture since decentering sensitivitybecomes big. Therefore, constituting an image forming optical systemwith four pieces of lens is best for performance as well as sizetogether.

[0025] According to the present image forming optical system, chromatismcan be well corrected since it is constituted with a cemented lens by asecond lens having negative refracting power and a third lens havingpositive refracting power. Also, the total length can be shortened sincethe air space between the second lens and the third lens can beeliminated. In this image forming optical system, the surface of anobject side of the second lens and the center of curvature of thesurface of an image side of the third lens are directed toward theobject side. By such constitution an angle of deflection that is anangle made by an incident light and exiting light to the cemented lenscan be kept small. As a result, the generation of an aberration in eachrefracting surface can be made small to the utmost. Also, it becomespossible to make small to the utmost about performance change at thetime of the relative decentering of the lens because the generation ofthe aberration in case of non-decentering is small.

[0026] An aperture stop is arranged on the object side of the first lenshaving positive refracting power.

[0027] When using CCD for example as an imaging element in order tomaintain a good performance for collecting light, the incidence angle oflight to an imaging element must be made small. For this purpose, it isdesirable to arrange an aperture stop in a distant position from animage plane. Otherwise, it is desirable to form an image of the aperturestop in the distant position far from the image plane. Furthermore, byconstituting the aperture stop with movable parts, it is possible tochange F number and perform a role of shutter. However, in this case, itis difficult to arrange it between lenses from the viewpoint of securingspace. Accordingly, in this image forming optical system, the positionof the aperture stop is arranged at the object side of the first lenshaving positive power. With this arrangement, it is possible to make theaperture stop have multifunction. Moreover, with this arrangement, anoptical system in which importance of telecentric nature andproductivity are taken into consideration can be realized.

[0028] In this image forming optical system, in order to shorten thewhole length of the image forming optical system, the fourth lens havingnegative power is arranged. Here, for example, it is supposed that CCDis used as an imaging element to enable to limit an incidence angle inorder to avoid the shading. In this case, if the lens on the utmostobject side has negative power in a wide-angle optical system it isinconvenient because the incidence angle of the light cannot be madesmall at the position where the height of the light is big. Therefore,at least one surface of the lens on the utmost object side (the fourthlens) is aspherical. And then, the power of the peripheral portion ofthis aspherical surface is made to be small as much as possible,otherwise to be positive power to negative power of the center portion.By such way, the light at the position where the height of the light ishigh is widely refracted toward the optical axis side, and thus thelight incidence angle to the image plane can be small. Therefore, thefollowing condition is important for the lens at the utmost image planeside.

−1.0<φm/φp<0.25   (1)

[0029] where φm is the power of the lens at the position with themaximum height of the light and φp is the power of the lens at theparaxis. Here, the power of the lens φm in the position with maximumheight of the light is defined as follows. That is, it is given by φm=tan ξm, where Hm is the maximum height of the light of the lens and ξis an inclined angle of the light after passing through the lens when aparallel light is entered from the infinite point at the object side tothe maximum height of the light of the lens as an object.

[0030] When falling below the minimum of this condition (1), the powerof the paraxis becomes weak too much and the whole length has becomelong, otherwise, the positive power of circumference becomes strong toomuch. Thus, the performance of circumference is remarkably degraded. Onthe other hand, when exceeding the upper limit, the positive power ofcircumference of the lens becomes weak too much and the correction ofthe incident angle of the light to the image plane becomes insufficient.

[0031] It is better to satisfy the following condition (1′).

−0.5<φm/φp<0.20   (1′)

[0032] It is much better to satisfy the following condition (1″).

−0.2<φm/φp<0.18   (1″)

[0033] The image forming optical system comprise in order from theobject side, a first lens having positive refracting power, a secondlens having negative refracting power, a concave surface of which isdirected toward the object side, a third lens having positive refractingpower, a convex surface of which is directed toward an image side and afourth lens having negative refracting power, wherein the first lensconsists of glass and the second lens and the third lens are cemented.

[0034] It is desirable that both of refracting surfaces of the firstlens having positive refracting power are spherical surface.

[0035] When making an optical surface aspherical surface, there aremethods of manufacture by grinding and a method of manufacture bymolding formation. In the former case, much manpower-day is needed andmoreover brings a welter to the refracting surface. Therefore, a goodoptical performance cannot be obtained. As for the latter, manpower-dayis not necessary for the formation. However, since it takes cost andtime for processing a die for the molding, it is hard to be referred asbeing cheap. Moreover, the surface precision as much as the conventionalpolished spherical surface cannot be obtained and a good opticalperformance cannot be achieved.

[0036] By constituting the first lens with glass lens, it is possible tomanufacture it by polishing process which is low cost.

[0037] A similar effect can be seen by constituting only with sphericalsurface. By constituting as mentioned above, a high precision lens canbe manufactured. Thus, a good optical performance can be obtained asmentioned above.

[0038] Glass lens is more resistant against changes of refracting indexand its volume by temperature change and humidity change than plasticlens. Therefore, it is possible to suppress degrading of the performanceof whole image forming optical system by concentrating the power to thefirst lens since the first lens consists of glass. Therefore, it isdesirable to satisfy the following condition (2).

0.4<f/f 1<2.0   (2)

[0039] where f is the focal length of the whole image forming opticalsystem and f1 is the focal length of the first lens.

[0040] When exceeding the upper limit of this condition (2), by causingto increase excessively the power of the first lens, it becomesnecessary to increase powers of other lenses and will cause generationof aberration and increase of decentering sensitivity. On the otherhand, when falling below the minimum, the power of the first lensbecomes too much small and the quantity of degradation of performancebecomes big to the temperature and the humidity change.

[0041] It is better to satisfy the following condition.

0.6<f/f 1<1.5   (2′)

[0042] It is much better to satisfy the following condition.

0.8<f/f 1<1.2   (2″)

[0043] A cemented lens of the second lens and the third lens which ispositioned in the middle of the image forming optical system is the lenswhich becomes a factors for generating coma, curvature of field andastigmatism. Therefore, to keep small as to the deflection angle, thatis, the angle made by the incident light and the exiting light to thecemented lens is important in order to reduce generation of suchaberrations as much as possible. By the reason mentioned above, it ispossible to make small to the utmost with respect to the performancechange when lenses are in a relative decentering position because thegeneration rate of aberrations when they are not in decentering positionis small.

[0044] It is good to satisfy the following condition (3) from thesereasons.

0.5<r2f/r3r<4.0   (3)

[0045] where r2f is a curvature radius on the side of the object of thesecond lens and r3r is a curvature radius on the side of the image ofthe third lens.

[0046] When exceeding the upper limit or falling below a minimum limitof this condition (3), the deflection angle becomes big too much and itbecomes difficult to correct the aberrations generated in the cementedlens, by the first lens and the fourth lens. Therefore, the good opticalperformance cannot be obtained, and moreover the decentering sensitivitybecomes big and the degree of difficulty in manufacture increases.

[0047] It is better to satisfy the following condition.

1.0<r2f/r3r<3.0   (3′)

[0048] It is much better to satisfy the following condition.

1.3<r2f/r3r<2.5   (3″)

[0049] In the image forming optical system, in order to shorten thetotal length, a telephoto type optical system is constituted so that thecomposite power of the first lens and the cemented lens consisting ofthe second lens and the third lens has positive power, and the power ofthe fourth lens is made to be negative. Therefore, in order to achievewell-balanced relation between the total length and a performance inthis telephoto type arrangement where the positive power and thenegative power are arranged, it is good to satisfy the followingconditions (4) and (5).

0.3<f123/|f4|<2.0   (4)

0.5<f/|f4|<2.0   (5)

[0050] where f123 is the composite focal length of the first lens andthe cemented lens consisting of the second lens and the third lens, f4is the focal length of the fourth lens and f is the focal length of thewhole image forming optical system.

[0051] When missing from the above conditions (4), (5), the balance ofthe positive power and the negative power which constitute the telephototype system collapses and this causes increase of the whole length anddegradation of the performance. That is, when exceeding the upper limitof the conditions above mentioned, it is disadvantageous for shorteningthe total length because the negative power the telephoto type systembecomes weak. On the other hand, when falling below the minimum limit,the negative power in the telephoto type system becomes strong too much.Accordingly the positive power must be increased and aberrationsgenerated in each lens increase. Therefore, it becomes difficult tosecure performance.

[0052] It is better to satisfy the following conditions.

0.4<f123/|f4|<1.5   (4′)

0.6<f/|f4|<1.7   (5′)

[0053] It is much better to satisfy the following conditions.

0.5<f123/|f4|<1.0   (4″)

0.8<f/|f4|<1.4   (5″)

[0054] When using CCD as an imaging element, if off-axial luminous fluxexited from an image forming optical system enters into an image planeat too big incident angle, a phenomenon so-called the shading occurs,where the brightness of a picture differs at the center of the pictureand at the circumference of the picture. On the other hand, if anincident angle to the image plane is small, this problem is reduced.However, in this case, the total length of image forming optical systembecomes longer. Therefore, it is good to satisfy the following condition(6).

0.6<EXP/f<2.0   (6)

[0055] Where EXP is the distance from an image plane to an exit pupiland f is the focal length of the image forming optical system as awhole.

[0056] When exceeding the upper limit of this condition (6), the totallength becomes long. On the other hand, when falling below the minimumlimit the incident angle to CCD becomes too big, and the brightness atthe circumference of the picture decreases.

[0057] So, it is good to satisfy the following condition (6′).

0.8<EXP/f<1.7   (6′)

[0058] It is better to satisfy the following condition (6″).

1.0<EXP/f<1.4   (6″)

[0059] It is good to satisfy the following condition (7), where Fno. isfully opened f-number of the optical system and P is a pitch of apicture element of the imaging element.

0.40 [1/μm]<Fno/P[μm]<2.20 [1/μm]  (7)

[0060] When exceeding the upper limit of this condition (7), thequantity of light per pixel decreases because the optical system becomestoo dark or the pitch of the picture element becomes too much small.Therefore, the shutter speed becomes slow and this causes a hand blurand increase of noise owing to long exposure time. On the other hand,when falling below the minimum limit, the pitch becomes too big to beunable to get high pixel image data.

[0061] It is better to satisfy the following condition (7′).

0.55 [1/μm]<Fno/P[μm]<1.50 [1/μm]  (7′)

[0062] It is much better to satisfy the following condition (7″).

0.77 [1/μm]<Fno/P[μm]<1.18 [1/μm]  (7″)

[0063] Then, when TL is the full length of an optical system and ML isthe minimum thickness of a plastic lens on the axis, it is good tosatisfy the following condition (8).

0.045<ML/TL<0.100   (8)

[0064] When exceeding the upper limit of this condition (8), theworkability of a glass lens is aggravated, as the thickness of center ofthe glass lens cannot be sufficiently secured because the minimumthickness of a plastic lens on the axis is too big to the total length.On the other hand, when falling below the minimum limit, plastic resincannot be entered smoothly into a molding die because the minimumthickness of a plastic lens on the axis is too small. Accordingly itcauses generating stress and double refraction, and loweringproductivity since much longer time is necessary for molding.

[0065] It is better to satisfy the following condition (8′).

0.055<ML/TL<0.085   (8′)

[0066] It is much better to satisfy the following condition (8″)

0.067<ML/TL<0.072   (8″)

[0067] Then, it is good to satisfy the following condition (9), where Rcis a curvature radius of a cemented surface of the cemented lens andRave is an average value of the curvature radius on the incident sideand the curvature radius on the exiting side.

−0.30<Rave/Rc<0.15   (9)

[0068] When exceeding the upper limit of this condition (9) theachromatic function does not work since the power of both lensescemented becomes too weak as the value of the curvature radius of thecemented surface becomes closely to these of the incident surface andthe exiting surface. On the other hand, when falling below the minimumlimit, to the contrary, the correction of chromatism becomes tooexcessive because the power of two cemented lenses becomes too strong.

[0069] It is better to satisfy the following condition (9′).

−0.20<Rave/Rc<0.10   (9′)

[0070] It is much better to satisfy the following condition (9″).

−0.12<Rave/Rc<0.06   (9″)

[0071] Hereinafter, embodiments of the image forming optical systemaccording to the present invention will be explained in detail,referring to FIG. 1 to FIG. 4.

First Embodiment

[0072]FIG. 1 is a sectional view showing an optical arrangementdeveloped along the optical axis in the first embodiment of an imageforming optical system.

[0073] In FIG. 1, an image forming optical system of the firstembodiment comprises, in order from an object side A toward an imagingelement surface P, an aperture stops S, a first lens L11 havingdouble-convex surfaces, a second lens L12 having negative refractingpower, a concave surface of which is directed toward an object side, athird lens having positive refracting power, a convex surface of whichis directed toward an image side, a fourth lens L14 which is a meniscuslens having negative refracting power, a convex surface of which isdirected toward the object side and filter members FL. The second lensL12 and the third lens L13 are cemented.

[0074] In this embodiment, the first lens L11 consists of glass. Thesecond lens L12, the third lens L13 and the fourth lens L14 are made ofplastic. The both surfaces of the first lens L11 are spherical. Theobject surface side of the second lens L12, the image surface side ofthe third lens L13 and the both surfaces of the fourth lens L14 areaspherical. An optical system which is strong against environment changeis achieved by using polished glass as both spherical surfaces of thefirst lens L11 having strong power.

[0075] As plastic material to be used here, polycarbonate material isused for the second lens L12 and the fourth lens L14, and Zeonex whichis polyolefin material is used for Lens L13 having positive refractingpower. In the image forming optical system of this embodiment, focallength is 4.65 mm, F number is 2.8, image height HT is 3.0 mm and halffield angle ω is 33° whereby a wide angle image forming optical systemis constituted.

[0076] On the image plane of the optical system, an imaging elementhaving 1,300,000 pixels (a pitch of picture element 3.6 μm) in ⅓ inchessquare is arranged.

[0077] Lens data of optical members constituting the image formingoptical system of the first embodiment are listed below.

[0078] In the numerical data, r₁, r₂, - - - denote radii of curvature ofindividual lens surfaces; d₁, d₂, - - - denote thickness of individuallenses or air space between them; n_(d1), n_(d2), - - - denoterefractive indices of individual lenses at the d line; v_(d1),v_(d2), - - - denotes Abbe's numbers of individual lenses; Fno. denotesan F number; f denotes total focal length of the image forming opticalsystem, and D0 denotes distance from an object to the first surface of alens element.

[0079] Also, when z is taken as the coordinate in the direction of theoptical axis, y is taken as the coordinate normal to the optical axis, Krepresents a conic constant, and A₄, A₆, A₈, and A₁₀ representaspherical coefficients, the configuration of each of the asphericalsurface is expressed by the following equation:

z=(y ² /r)/[1+{1−(1+K)(y/r)²}^(1/2) ]+A ₄ y ⁴ +A ₆ y ⁶ +A ₈ y ⁸ +A ₁₀ y¹⁰

[0080] These symbols hold for the numerical data of embodiments to bedescribed later. numerical data 1 focal length f = 4.65 mm, Fno. = 2.8,image height HT = 3.0 mm, half field angle ω = 33°, distance from anobject DO = ∞ r₁ = ∞ (aperture stop) d₁ = 0.00 r₂ = 2.68 d₂ = 0.90n_(d2) = 1.5163 ν_(d2) = 64.1 r₃ = −21.36 d₃ = 0.77 r₄ = −1.96(aspherical surface; the d₄ = 0.50 n_(d4) = 1.5839 ν_(d4) = 30.2 fourthsurface) r₅ = −30.60 d₅ = 1.47 n_(d5) = 1.5091 ν_(d5) = 56.2 r₆ = −1.18(aspherical surface; the d₆ = 0.05 sixth surface) r₇ = 2.21 (asphericalsurface; the d₇ = 0.65 n_(d7) = 1.5839 ν_(d7) = 30.2 seventh surface) r₈= 1.03 (aspherical surface; the d₈ = 1.16 eighth surface) r₉ = ∞ d₉ =1.00 n_(d9) = 1.5163 ν_(d9) = 64.1 r₁₀ = ∞ d₁₀ = 0.50 r₁₁ = ∞ (imageplane) aspherical surface coefficient the fourth surface K = −6.2276 A₂= 0 A₄ = −7.2439 × 10⁻² A₆ = 3.0016 × 10⁻² A₈ = −3.716 × 10⁻³ A₁₀ =−1.4490 × 10⁻⁴ the sixth surface K = −3.8561 A₂ = 0 A₄ = −5.6955 × 10⁻²A₆ = 2.3202 × 10⁻² A₈ = −3.6193 × 10⁻³ A₁₀ = 3.8559 × 10⁻⁴ the seventhsurface K = −8.5014 A₂ = 0 A₄ = −3.7239 × 10⁻³ the eighth surface K =−5.0721 A₂ = 0 A₄ = −1.3207 × 10⁻² A₆ = 4.3425 × 10⁻⁴

Second Embodiment

[0081]FIG. 2 is a sectional view showing an optical arrangementdeveloped along the optical axis in the second embodiment of an imageforming optical system.

[0082] In FIG. 2, an image forming optical system of the secondembodiment comprises in order from an object side A toward an imagingelement plane P, an aperture stops S, a first lens L21 havingdouble-convex surfaces, a second lens L22 having negative refractingpower, a concave surface of which is directed toward an object side, athird lens L23 having positive refracting power, a convex surface ofwhich is directed toward an image side and, a fourth lens L24 which is ameniscus lens having negative refracting power, a convex surface ofwhich is directed toward the object side and filter members FL. Thesecond lens L22 and the third lens 23 are cemented.

[0083] In this embodiment, the first lens L21 consists of glass. Thesecond lens L22, the third lens L13 and the fourth lens L24 are made ofplastic. The both surfaces of the first lens L21 are a spherical. Theobject side surface of the second lens L22, the image side surface ofthe third lens L23 and the both surfaces of the fourth lens L24 areaspherical. An optical system which is strong against environment changecan be achieved by using polished glass as both surfaces of the firstlens L21 having strong power. As plastic materials to be used here,polycarbonate material is used for the second lens L22 and the fourthlens L24, and Zeonex which is polyolefin material is used for Lens L23.In the image forming optical system of this embodiment, focal length is4.62 mm, F number is 2.8, image height HT is 3.0 mm and half field angleω is 33° whereby a wide angle image forming optical system isconstituted. On the image plane of the optical system, an imagingelement having 2,000,000 pixels (a pitch of picture element 3.0 μm) in ⅓inches square is arranged. numerical data 2 focal length f = 4.62 mm,Fno. = 2.8, image height HT = 3.0 mm, half field angle ω = 33°, distancefrom an object DO = ∞ r₁ = ∞ (aperture stop) d₁ = 0.00 r₂ = 2.85 d₂ =0.89 n_(d2) = 1.5163 ν_(d2) = 64.1 r₃ = −20.08 d₃ = 0.67 r₄ = −2.42(aspherical surface; the d₄ = 0.50 n_(d4) = 1.5839 ν_(d4) = 30.2 fourthsurface) r₅ = 23.95 d₅ = 1.57 n_(d5) = 1.5091 ν_(d5) = 56.2 r₆ = −1.24(aspherical surface; the d₆ = 0.05 sixth surface) r₇ = 1.66 (asphericalsurface; the d₇ = 0.49 n_(d7) = 1.5839 ν_(d7) = 30.2 seventh surface) r₈= 0.89 (aspherical surface; the d₈ = 1.33 eighth surface) r₉ = ∞ d₉ =1.00 n_(d9) = 1.5163 ν_(d9) = 64.1 r₁₀ = ∞ d₁₀ = 0.50 r₁₁ = ∞ (imageplane) aspherical surface coefficient the fourth surface K = −8.6660 A₂= 0 A₄ = −5.9446 × 10⁻² A₆ = 2.3227 × 10⁻² A₈ = −4.5130 × 10⁻³ A₁₀ =7.0562 × 10⁻⁴ the sixth surface K = −4.2776 A₂ = 0 A₄ = −5.2010 × 10⁻²A₆ = 1.8759 × 10⁻² A₈ = −3.3248 × 10⁻³ A₁₀ = 4.0585 × 10⁻⁴ the seventhsurface K = −5.6700 A₂ = 0 A₄ = −9.5173 × 10⁻³ the eighth surface K =−3.7822 A₂ = 0 A₄ = −1.2552 × 10⁻² A₆ = 2.3741 × 10⁻⁴

Third Embodiment

[0084]FIG. 3 is a sectional view showing an optical arrangement,developed along the optical axis in the third embodiment of an imageforming optical system.

[0085] In FIG. 3, an image forming optical system of the thirdembodiment comprises in order from an object side A toward an imagingelement plane P, an aperture stops S, a first lens L31 havingdouble-convex surfaces, a second lens L32 having negative refractingpower a concave surface of which is directed toward an object side, athird lens L33 having positive refracting power, a convex surface ofwhich is directed toward an image side and, a fourth lens L34 which is ameniscus lens having negative refracting power, a convex surface ofwhich is directed toward the object side, and filter members FL. Thesecond lens L32 and the third lens 33 are cemented. In this embodiment,the first lens L31 consists of glass. The second lens L32, the thirdlens L33 and the fourth lens L34 are made of plastic. The both surfacesof the first lens L31 are aspherical. The object side surface of thesecond lens L32, the image side surface of the third lens L33 and theboth surfaces of the fourth lens L24 are aspherical. An optical systemwhich is strong against environment change can be achieved by usingpolished glass as both surfaces of the first lens L31 having strongpower. As plastic materials to be used here, polycarbonate material isused for the second lens L32 and the fourth lens L34, and Zeonex whichis polyolefin material is used for Lens L33. In the image formingoptical system of this embodiment, focal length is 4.57 mm, F number is2.4, image height HT is 3.0 mm and half field angle ω is 33° whereby awide angle image forming optical system is constituted.

[0086] On the image plane of the optical system, an imaging elementhaving 3,000,000 pixels (a pitch of picture element 2.4 μm in ⅓ inchessquare is arranged. numerical data 3 focal length f = 4.57 mm, Fno. =2.8, HT = 3.0 mm, half field angle ω = 33°, distance from an object DO =∞ r₁ = ∞ (aperture stop) d₁ = 0.00 r₂ = 3.16 d₂ = 0.9 ν_(d2) = 59.4second₂ = 1.5831 r₃ = −30.42 d₃ = 0.62 r₄ = −2.55 (aspherical surface;the d₄ = 0.50 n_(d4) = 1.5839 ν_(d4) = 30.2 fourth surface) r₅ = 17.65d₅ = 1.60 n_(d5) = 1.5091 ν_(d5) = 56.2 r₆ = −1.32 (aspherical surface;the d₆ = 0.05 sixth surface) r₇ = 1.54 (aspherical surface; the d₇ =0.48 n_(d7) = 1.5839 ν_(d7) = 30.2 seventh surface) r₈ = 0.88(aspherical surface; the d₈ = 1.34 eighth surface) r₉ = ∞ d₉ = 1.00n_(d9) = 1.5163 ν_(d9) = 64.1 r₁₀ = ∞ d₁₀ = 0.50 r₁₁ = ∞ (image plane)aspherical surface coefficient the fourth surface K = −1.1792 A₂ = 0 A₄= −6.7015 × 10⁻² A₆ = 2.9979 × 10⁻² A₈ = −6.3844 × 10⁻³ A₁₀ = 8.1531 ×10⁻⁴ the sixth surface K = −4.4150 A₂ = 0 A₄ = −4.5570 × 10⁻² A₆ =1.4393 × 10⁻² A₈ = −2.2759 × 10⁻³ A₁₀ = 3.0507 × 10⁻⁴ the seventhsurface K = −4.3210 A₂ = 0 A₄ = −1.2167 × 10⁻² the eighth surface K =−3.4003 A₂ = 0 A₄ = −1.3540 × 10⁻² A₆ = 6.7047 × 10⁻⁵

Fourth Embodiment

[0087]FIG. 4 is a sectional view showing an optical arrangement,developed along the optical axis in the fourth embodiment of an imageforming optical system.

[0088] In FIG. 4, an image forming optical system of the fourthembodiment comprises in order from an object side A toward an imagingelement plane P, an aperture stops S, a first lens L41 havingdouble-convex surfaces, a second lens L42 having negative refractingpower a concave surface of which is directed toward an object side and,a third lens L43 having positive refracting power, a convex surface ofwhich is directed toward an image side and, a fourth lens L44 which is ameniscus lens having negative refracting power, a convex surface ofwhich is directed toward the object side and filter members FL. Thesecond lens L42 and the third lens 43 are cemented. In this embodiment,all lenses are made of plastic. The both surfaces of the first lens L41are spherical. The object side surface of the second lens L42, the imageside surface of the third lens L43 and the both surfaces of the fourthlens L44 are aspherical. An optical system which is strong againstenvironment change can be achieved if polished glass with doublespherical surfaces is used for the first lens L41 having strong power.As plastic materials to be used here, polycarbonate material is used forthe second lens L42 and the fourth lens L44, and Zeonex which ispolyolefin material is used for Lens L33. In the image forming opticalsystem of this embodiment, focal length is 4.79 mm, F number is 2.8,image height HT is 3.0 mm and half field angle ω is 32° whereby a wideangle optical imaging system is constituted.

[0089] On the image plane of the optical system, an imaging elementhaving 1,300,000 pixels (a pitch of picture element 3.6 μm) i n ⅓ inchessquare is arranged numerical data 4 focal length f = 4.79 mm, Fno. =2.8, image height HT = 3.0 mm, half field angle ω = 32°, distance froman object DO = ∞ r₁ = ∞ (aperture stop) d₁ = 0.00 r₂ = 2.55 d₂ = 0.93n_(d2) = 1.5091 ν_(d2) = 56.2 r₃ = −21.90 d₃ = 0.81 r₄ = −1.95(aspherical surface; the d₄ = 0.50 n_(d4) = 1.5839 ν_(d4) = 30.2 fourthsurface) r₅ = 508.05 d₅ = 1.46 n_(d5) = 1.5091 ν_(d5) = 56.2 r₆ = −1.17(aspherical surface; the d₆ = 0.05 sixth surface) r₇ = 2.31 (asphericalsurface; the d₇ = 0.67 n_(d7) = 1.5839 ν_(d7) = 30.2 seventh surface) r₈= 1.01 (aspherical surface ; the d₈ = 1.08 eighth surface) r₉ = ∞ d₉ =1.00 n_(d9) = 1.5163 ν_(d9) = 64.1 r₁₀ = ∞ d₁₀ = 0.50 r₁₁ = ∞ (imageplane) aspherical surface coefficient the fourth surface K = −9.3675 A₂= 0 A₄ = −1.1587 × 10⁻¹ A₆ = 7.6878 × 10⁻² A₈ = −2.9460 × 10⁻² A₁₀ =5.0577 × 10⁻³ the sixth surface K = −3.9196 A₂ = 0 A₄ = −5.8984 × 10⁻²A₆ = 2.6782 × 10⁻² A₈ = −4.0895 × 10⁻³ A₁₀ = 3.6299 × 10⁻⁴ the seventhsurface K = −1.3717 A₂ = 0 A₄ = −2.5298 × 10⁻³ the eighth surface K =−5.8628 A₂ = 0 A₄ = −1.4946 × 10⁻² A₆ = 6.1189 × 10⁻⁴

[0090] In the embodiments mentioned above, a part of or all of lensesare made of plastic. However, it is possible to be constituted withglass instead of plastic. If lenses of an image forming optical systemare constituted with glass, it is possible to make an image formingoptical system which is strong against the change of the temperature andthe humidity. If using glass with refractive index which is higher thanthat of the material used in this embodiment, it is possible to make animage forming optical system having higher performance. If using speciallow dispersion glass, it is effective for correction of the chromatism.When constituting a lens by plastic, degradation of the performance byenvironment change can be reduced by using low hygroscopic material.

[0091] In order to cut an unnecessary light of ghost, flare and thelike, a flare cut stop may be used instead of an aperture stop S. Thisflare cut aperture may be arranged in any place which is either in frontof the first lens, between the second lens and the third lens, betweenthe third lens and the fourth lens, or between the fourth lens and thesurface of an imaging element. To make use of the function of flare cutstop, the way of cutting a flare light by using a frame may be adopted.The way for cutting flare light by providing another part may be alsoused as another way. Also, it is possible to constitute a flare cutaperture by printing, painting and gluing a seal and the like, directlyto the image forming optical system.

[0092] As to the shape of the stop, any type of shape formed by such asa circle, an ellipse, a rectangle, a polygon and a scope surrounded by afunction curve can be also used. For this purpose, these may be not onlyfor cutting detrimental light but also cutting light of the coma flarearound the picture plane. Moreover, in order to reduce ghost and flare,coating for preventing reflection can be made to each lens. By usingmultiple coating the ghost and the flare can be efficiently reduced.Infrared cut coat can be also made to surfaces of a lens and a coverglass and the like.

[0093] Furthermore, in order to adjust focus regulation a focusing canbe carried out. As focusing method, there are a type where the wholelenses or a part of lenses is moved outward, and other type where thewhole lenses or a part of lenses is moved inward.

[0094] By shifting a micro lens of CCD, decrease of the brightnessaround circumference of picture plane can be reduced. For example, thedesign of the micro lens of CCD may be changed according to theincidence angle of the light in each image height. Correction ofdecreased quantity of the brightness around circumference of a pictureplane can be carried out by image processing.

[0095] Numerical values calculated by conditions mentioned aboveconcerning each embodiment from the first embodiment to the fourthembodiment are shown in the following table. first second third fourthembodiment embodiment embodiment embodiment φm/φp −0.11 0.13 0.14 −0.14f/f1 1.00 0.95 0.92 1.06 r2f/r3r 1.66 1.95 1.94 1.66 f123/|f4| 0.77 0.730.68 0.85 f/|f4| 1.12 1.08 0.96 1.26 EXP/f 1.26 1.29 1.29 1.17 Fno/P[μm]0.78 0.93 1.17 0.78 ML/TL 0.071 0.070 0.068 0.071 Rave/Rc 0.051 −0.076−0.110 −0.003

[0096] The image forming optical system according to the presentinvention is suitable for an optical apparatus which is used forelectronic instrument such as cameras, cellular phones, portable typeinformation entry terminal and the like using a film and CCD as arecording part.

[0097]FIG. 9 shows outlined construction of an electronic instrument ofthe first embodiment according the present invention. This embodimentshows an example where an image forming optical system of the presentinvention is applied to a cellular phone. FIG. 9A is a front view andFIG. 9B is a rear view showing appearance of the cellular phone.

[0098] In FIG. 9, reference numeral 1 represents an antenna fortransmitting and receiving an electric wave, 2 represents a displayportion such as LCD, 3 is a speaker portion for catching sound, 4 is anoperation portion, 5 is a microphone portion, 6 is an image formingoptical system portion including an image forming optical system of thepresent invention, which is arranged at the opposite side to the saidportions and 7 is a battery and 8 is a back side monitor.

[0099]FIG. 10 shows outlined construction of a digital camera in whichan image forming optical system according to the present invention isused in a photographing optical system. FIG. 10A is a front perspectiveview showing appearance of the digital camera and FIG. 9B is a rearperspective view of the same.

[0100]FIG. 10, reference numeral 11 represents a photographing opticalsystem having photographing optical path 12, reference numeral 13 is afinder optical system with an optical path 14 for finder, 15 is ashutter button, 16 is a flush lump and 17 is a monitor with liquidcrystal display. When pushing the shutter button 15 arranged on thecamera, in responding such action photographing is carried out via thephotographing optical system 11.

What is claimed is:
 1. An imaging optical system comprising: in order from an object side, a first lens having positive refracting power, a second lens having negative refracting power, a concave surface of which is directed toward the object side, a third lens having positive refracting power, a convex surface of which is directed toward an image side and a fourth lens having negative refracting power, wherein the second lens and the third lens are cemented.
 2. An imaging optical system according to claim 1 comprising: an aperture stop which is arranged at the object side of the first lens.
 3. An imaging optical system comprising: in order from an object side, a first lens having positive refracting power, a second lens having negative refracting power, a concave surface of which is directed toward the object side, a third lens having positive refracting power, a convex surface of which is directed toward an image side and a fourth lens having negative refracting power, wherein the first lens consists of glass and the second lens and the third lens are cemented.
 4. An imaging optical system according to claim 1: wherein at least one of surfaces of the fourth lens is aspherical and the following condition is satisfied: −1.0<φm/φP<0.25 Where φm represents the power of the lens at the position of the maximum light height and φp represents the power of the lens at the position of the paraxis.
 5. An imaging optical system according to claim 3, wherein both refracting surfaces of the first lens are spherical.
 6. An imaging optical system according to claim 3, wherein the following condition is satisfied: 0.4<f/f1<2.0 where f represents the focal length of the whole optical system and f1 represents the focal length of the first lens.
 7. An imaging optical system according to claim 1, wherein the following condition is satisfied: 0.5<r2f/r3r<4.0 Where r2f represents the radius of curvature of the second lens at the object side and r3r represents the radius of curvature of the third lens at the image side.
 8. An imaging optical system according to claim 1, wherein the second lens and the third lens are cemented, and the following conditions are satisfied: 0.3<f123/|f4|<2.0 0.5<f/|f4|<2.0 where f123 represents composite focal length of the cemented lens consisting of the first, the second and the third lens, f4 represents the focal length of the fourth lens, and f represents the focal length of whole optical system.
 9. An imaging optical system according to claim 1, wherein the following condition is satisfied: 0.6<EXP/f<2.0 where EXP represents the length from an object plane to an exit pupil and f represents the focal length of whole optical system.
 10. An electronic instrument comprising the imaging optical system according to claim 1,
 11. An imaging optical system according to claim 1 or 3, wherein the following condition is satisfied: 0.40(1/μm)<Fno/P(μm)<2.20(1/μm) where Fno represents the F number fully opened and P represents the pitch of an imaging element.
 12. An imaging optical system according to claim 3, wherein the following condition is satisfied: 0.045<ML/TL<0.100 where TL represents whole length of the optical system and ML represents the minimum thickness on the axis of a plastic lens.
 13. An imaging optical system according to claim 1 or 3, wherein the following condition is satisfied: −0.30<Rave/Rc<0.15 where Rc represents the radius of curvature of the cemented surface of the cemented lens and Rave represents an average value of the radius of curvature of incident side and that of exit side. 